American Chemical Society
The Journal of Physical Chemistry C
Excimer formation and charge resonance stabilization in covalently linked bichromophoric systems with flexible spacers are important processes relevant to biochemistry and functional materials. Requiring a π-stacked cofacial arrangement of a pair of aromatic molecules at a van der Waals contact, the underlying geometrical reorganization that accompanies these events continues to be debated. Here we use a variety of methods including two-color resonant two-photon ionization spectroscopy (2CR2PI), ion yield measurements, hole-burning spectroscopy (HB), and laser-induced fluorescence (LIF) excitation and emission spectroscopy to compare the gas-phase spectroscopy and dynamics of the van der Waals dimers of fluorene, 9-methylfluorene (MF), and 9,9′-dimethylfluorene (F1). The goal of this work is to probe the influence of methyl substitution on the dynamics of excimer formation and charge resonance (CR) stabilization. The fluorene dimer, (F)2, displays lifetime broadened electronic spectra and the dominance of excimer emission, consistent with a rapid (picoseconds) formation of a π-stacked excimer upon electronic excitation. Ion yield measurements of (F)2 reveal a lowering of the ionization potential (IP) by some 0.38 eV relative to the monomer, reflecting significant CR stabilization. These trends are mirrored in the 9-methylfluorene dimer, (MF)2, as one face of the π-system remains open. In contrast, the electronic spectrum of the dimethyl-substituted dimer, (F1)2, shows narrow features representing a single band system, and analysis of the torsional structure in dispersed fluorescence spectra identifies this as emission from the locally excited state of a tilted (non-π-stacked) dimer, with no evidence of excimeric emission. The structure of this dimer reflects the increased importance of C–H/π interactions in the dimethyl-substituted system, as increased steric constraints block a cofacial approach. The IP of (F1)2 shows CR stabilization which is roughly 1/2 of that in π-stacked (F)2 dimer. Extensive theoretical calculations support these findings and show the importance of sandwich-type configurations for excitonic delocalization and CR stabilization.